Continuous monitoring of left ventricular (LV) function during percutaneous transluminal coronary angioplasty (PTCA) was performed in 40 patients (53 ± 2 years) with a miniature, nuclear detector system after labeling the patients' red blood cells with technetium-99m. Balloon dilation (113 second, range 60 to 240) induced on average a 0.12 ejection fraction (EF) unit (19%) decrease in the LVEF, which was explained by a 34% increase in end-systolic counts. Balloon dilation of the left anterior descending artery (n = 23) produced a decrease in the LVEF of 0.17 ± 0.13 EF units compared with the decrease of 0.06 ± 0.07 EF units in patients undergoing dilation of the left circumflex artery (n = 9) and 0.05 ± 0.04 EF units in patients treated for a stenosis of the right coronary artery (n = 8), (p = 0.02). Balloon deflation was associated with an immediate return to pre-PTCA levels. In 10 patients with 2 identical balloon occlusions, the second occlusion led to a significantly less decrease in the LVEF (0.41 ± 0.14 vs 0.44 ± 0.15) and electrocardiographic ST-segment deviation (88 ± 54 μV vs 65 ± 42 μV) than the first. We conclude that PTCA is associated with an abrupt transient decrease in the LVEF. The effect of balloon occlusion of the left anterior descending artery is more pronounced than balloon occlusion of the left circumflex and the right coronary arteries. Neither single nor multiple balloon occlusions were associated with post-PTCA global LV dysfunction, whereas the lesser degree of LV dysfunction and electrocardiographic signs of myocardial ischemia during the second of 2 identical balloon occlusions suggests that preconditioning can be induced during PTCA.